1. Field of the Invention
The present invention relates to a rotation-position detection system for a brushless motor in which a resolver is mounted.
2. Description of the Related Art
In the case of a control system for controlling a brushless motor, in order to optimally control the brushless motor, there is required a means for detecting the rotation position of the brushless motor. As means for detecting the rotation position of a brushless motor, a hole IC, a resolver, and the like are commonly known. Because it can more accurately detect the rotation position of a brushless motor than a hole IC, the resolver is suitable for a system for suppressing torque fluctuation while the motor rotates or a system that performs servo control.
With regard to a method of detecting the rotation position of a brushless motor by use of a resolver, Japanese Patent Application Laid-Open No. 2005-210839 discloses a method utilizing a dedicated resolver/digital conversion IC and a method utilizing a microcomputer. FIG. 12 represents a timing chart, described in Japanese Patent Application Laid-Open No. 2005-210839, at a time when the rotation position of a brushless motor is detected. In a method disclosed in Japanese Patent Application Laid-Open No. 2005-210839, a microcomputer supplies an excitation signal to a resolver; the microcomputer applies an analogue/digital conversion (referred to also as an A/D conversion, hereinafter) concurrently to a sine wave signal and a cosine wave signal that are outputted by the resolver, due to the supply of the excitation signal to the resolver; from the respective maximum values and minimum values of the foregoing signals, a sine wave signal amplitude V sin and a cosine wave signal amplitude V cos of the resolver are obtained; then, a pivoting angle θ of the resolver rotation axle is given by Equations (1) and (2) below.tan θ=(V sin)/(V cos)  (1)θ=arctan(V sin/V cos)  (2)
As an example of application of a method utilizing a brushless motor, there is known a system, as an electric power steering apparatus for an automobile, that controls the torque of a motor so as to assist the steering torque, as described in Japanese Patent No. 3427876. In addition, there is known a system, as the steering apparatus of a boat, that rotates an outboard engine mounted on a boat, as described in Japanese Patent No. 2959044.
In the conventional rotation-position detection system disclosed in Japanese Patent Application Laid-Open No. 2005-210839, for the purpose of accurately detecting the rotation position of a motor, it is required to apply the A/D conversion to the sine wave signal and the cosine wave signal outputted by the resolver at a single and the same timing; therefore, there is required a rotation-position detection system in which two or more analogue/digital converters (referred to also as A/D converters, hereinafter) are mounted. Alternatively, in the case where only a single A/D converter is mounted in the rotation-position detection system, it is required to provide a sample/hold circuit in an external peripheral circuit or the microcomputer, to apply sampling to the sine wave signal and the cosine wave signal at a single and the same timing, and then to apply the A/D conversion to the sampled signals. As described above, for the purpose of accurately detecting the rotation position of a brushless motor, it is required to mount a plurality of A/D converters or a sample/hold circuit on the rotation-position detection system; thus, there has been a problem that the system is expensive.
As described in Japanese Patent Application Laid-Open No. 2005-210839, it is generally required to perform processing of detecting the rotation position of a motor in a shorter control period than to perform control of the motor; in addition, in the case of the system described in Japanese Patent No. 3427876 or Japanese Patent No. 2959044, the control period for a brushless motor is fast (e.g., processed every 100 μs or 500 μs), in most cases. Accordingly, for the purpose of performing processing of detecting the rotation position of a brushless motor and processing of controlling the brushless motor, a high-performance (calculation processing is performed fast) microcomputer is required.
In the case of an electric power steering apparatus described in Japanese Patent No. 3427876, the torque assisted by a brushless motor is directly transferred to a steerer; therefore, it is required to accurately detect the rotation position of the brushless motor so as to control the torque. However, in the case of the steering system described in Japanese Patent No. 2959044, because the transfer mechanism for rotating an outboard engine and the steering system are mechanically separated, the torque of a brushless motor is not sensed by a steerer; thus, the rotation position of a brushless motor is not required to be as accurate as that of Japanese Patent No. 3427876.